This paper presents guidelines for determining the initial shape and specifications of a high-speed axial-flux permanent-magnet (AFPM) machine in a hybrid-electric propulsion system in two steps based on previous studies and product review results related to high-speed AFPMs. In the first step, three characteristics to be considered when designing AFPMs were classified as: electromagnetic, thermal, and mechanical. Then, the factors that should be considered in the design process to satisfy each characteristic were organized. In the second step, “the speed–output power” relationship was defined to predict the limits of applying AFPMs to high-speed applications, allowing an estimation of the limits of the speed range that can be used within the proposed output power.

This paper presents a refined thermal analysis of a semi-closed outer rotor BLDC motor designed for drone applications, with a focus on ventilation cooling effects induced by motor-generated airflow. To reduce weight, small drone motors are typically designed without dedicated cooling systems, relying instead on airflow generated during flight. Considering these operating conditions, thermal analysis that accurately reflects the airflow characteristics around the motor is essential in the motor design process. Existing thermal analysis methods each have limitations. Computational Fluid Dynamics (CFD) can analyze flow and heat transfer phenomena with high resolution but involves high computational cost, making it impractical for iterative design processes. Conversely, Lumped-Parameter Thermal Network (LPTN) models offer fast computation but require pre-defined convective heat transfer coefficients and have difficulty capturing complex flow conditions. In this study, a hybrid thermal analysis approach combining CFD and LPTN methods is proposed. First, motor losses under various load conditions are calculated using 2D and 3D electromagnetic Finite Element Analysis (FEA), incorporating the effect of axial leakage flux. Next, CFD simulations are performed on a coaxial rotor drone system to evaluate airflow characteristics near the motor. Based on the CFD results, convective heat transfer coefficients are determined and applied to the LPTN model, achieving both high accuracy and computational efficiency in thermal analysis. The proposed method was validated through prototyping and propeller load tests simulating various operating conditions. The comparison between the measured and analyzed winding temperatures showed good agreement, confirming the accuracy and reliability of the proposed approach. Notably, the results confirmed that most of the airflow entering the motor is generated by the motor’s own rotation rather than by the propeller, for both single-rotor and coaxial-rotor configurations. This CFD–LPTN-based thermal analysis framework provides a practical and validated method to evaluate the thermal performance of air-cooled drone motors under various operating conditions.

The increasing demand for lightweight, high-efficiency electrical machines has driven research into copper-carbon nanotube (Cu-CNT) composites as alternatives to copper windings. While Cu-CNT-based conductors show promise, their bi-directional electromagnetic-thermal behavior in permanent magnet synchronous generators (PMSGs) remains underexplored. This study evaluated the feasibility of Cu-CNT windings to enhance power density, thermal management, and efficiency through a coupled analysis using 2D electromagnetic finite element analysis integrated with a lumped parameter thermal network. Two Cu-CNT materials with a resistivity of $7.12\times 10^{-8}~\Omega $ .m (Cu-CNT #1) and $2.12\times 10^{-8}~\Omega $ .m (Cu-CNT #2) were tested over speeds ranging from 1000 to 4800 RPM with parametric variations in axial length and outer diameter. Results show that Cu-CNT #2 delivered higher specific power and, at high temperatures, achieved better efficiency than the copper-based model. Due to limited composite material samples from current processing and manufacturing, a copper coil prototype was manufactured for validation. The good agreement between the simulation and the test results of the copper-based confirms the accuracy of the coupled electromagnetic-thermal analysis. As a result, the correctness of the Cu-CNT-based model is validated.

This paper proposes a method for minimizing the inductor current ripple of a DC–DC converter in a two-stage power conversion system consisting of a grid-connected PWM converter and an interleaved multiphase three-level DC–DC converter. To reduce the output voltage ripple, the three-level DC–DC converter is configured in parallel and operated interleaved. However, a circulating current generated by the interleaved operation increases the inductor current ripple of each DC–DC converter and causes system loss and inductor saturation. In this paper, the inductor and output current ripple of the interleaved three-phase three-level DC–DC converter is mathematically analyzed and the effect of the DC–DC converter’s duty ratio and output voltage on each current ripple is described. Based on this analysis, a method is proposed for controlling the optimal DC link voltage through the PWM converter, so that the DC–DC converter is controlled with the duty ratio that minimizes the inductor current ripple. The simulation and experimental results under various operating conditions are presented to verify the feasibility of the proposed control method.

This study analyzes the material deformation of electrical steel with aging and its effects on the electromagnetic and thermal characteristics of a Brushless Wound-Field Synchronous Generator (BL-WFSG). First, in order to confirm the material deformation of electrical steel applied to the BL-WFSG, magnetic property tests are performed on the core sheets of the old and new generators. Those two generators are made of the same material, so there was no difference except for their usage time. Based on the results of the magnetic property tests, an electromagnetic field analysis is performed on the old and new generators, and analysis results are compared in order to confirm the effects of material deformation on the electromagnetic characteristics of BL-WFSG. Then, a thermal analysis is performed on the old and new generators using losses calculated by electromagnetic field analysis as heat sources, and analysis results are compared in order to confirm the effects of material deformation on the thermal characteristics of BL-WFSG. Finally, an additional electromagnetic analysis is performed on both the old and new generators, using the exact winding resistances that were calculated in the thermal analysis for each respective generator in order to calculate and compare the efficiency of the old and new generators. Through this process, the authors confirmed the effects of material deformation of the rotor and stator cores due to aging on the electromagnetic and thermal characteristics of the BL-WFSG.